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A World War is a war in which most of the countries in the world fight or help.
There have been two World Wars so far, World War I (1914-1918) and World War II (1939-1945). Several wars such as the Seven Years' War were fought in many places around the world, but they were wars among European countries.
After World War II, many people were scared that a World War III might start, probably between the United States and Western Europe on one side and the Soviet Union and maybe China on the other. After World War II, countries had many powerful weapons, including jet bomber airplanes, which could carry many powerful bombs fast and far; nuclear weapons which could destroy many buildings and kill many people instantly; and missiles which could carry nuclear weapons to other countries very fast.
Many people were afraid that World War III might start during the Cold War. After the end of the Soviet Union, people were less afraid that World War III would begin. However, many countries still have powerful weapons and can still use them.
Albert Einstein is supposed to have said, "I don't know what weapons countries might use to fight World War III, but wars after that will be fought with sticks and stones". That is because World War III would probably be so large that it would destroy civilization.